To get your firmware version on the psp goto
Settings>>
System Settings>>
System Information. That will give you you mac address, nickname and your firmware version.
Make sure your firmware is 2.0 and not 2.01 otherwise you cant downgrade, if it is 2.0 then here is some information on the downgrade process and the files you need.
http://forums.maxconsole.net/showthread.php?t=13733
I don't know anything about the actual process as I got a Japanese 1.0 PSP before the USA & UK launch and only had to update to 1.5 to get homebrew running.
Hope some of that helps.